DESCRIPTION

These functions shall convert the initial portion of the string pointed
to by nptr to double, float, and
long double representation, respectively. First, they decompose
the input string into three parts:

The subject sequence is defined as the longest initial subsequence
of the input string, starting with the first non-white-space
character, that is of the expected form. The subject sequence contains
no characters if the input string is not of the expected
form.

If the subject sequence has the expected form for a floating-point
number, the sequence of characters starting with the first
digit or the decimal-point character (whichever occurs first) shall
be interpreted as a floating constant of the C language, except
that the radix character shall be used in place of a period, and that
if neither an exponent part nor a radix character appears in
a decimal floating-point number, or if a binary exponent part does
not appear in a hexadecimal floating-point number, an exponent
part of the appropriate type with value zero is assumed to follow
the last digit in the string. If the subject sequence begins with
a minus sign, the sequence shall be interpreted as negated. A character
sequence INF or INFINITY shall be interpreted as an
infinity, if representable in the return type, else as if it were
a floating constant that is too large for the range of the return
type. A character sequence NAN or NAN(n-char-sequence_opt) shall
be interpreted as a quiet NaN, if
supported in the return type, else as if it were a subject sequence
part that does not have the expected form; the meaning of the
n-char sequences is implementation-defined. A pointer to the
final string is stored in the object pointed to by
endptr, provided that endptr is not a null pointer.

If the subject sequence has the hexadecimal form and FLT_RADIX is
a power of 2, the value resulting from the conversion is
correctly rounded.

The
radix character is defined in the program's locale (category LC_NUMERIC
). In the POSIX locale, or in a locale where the
radix character is not defined, the radix character shall default
to a period ( '.' ).

In other than the C or POSIX locales, other
implementation-defined subject sequences may be accepted.

If the subject sequence is empty or does not have the expected form,
no conversion shall be performed; the value of str
is stored in the object pointed to by endptr, provided that
endptr is not a null pointer.

The
strtod() function shall not change the setting of errno
if successful.

Since 0 is returned on error and is also a valid return on success,
an application wishing to check for error situations should
set errno to 0, then call strtod(), strtof(), or
strtold(), then check errno.

RETURN VALUE

Upon successful completion, these functions shall return the converted
value. If no conversion could be performed, 0 shall be
returned, and errno may be set to [EINVAL].

If the correct value is outside the range of representable values,
±HUGE_VAL, ±HUGE_VALF, or ±HUGE_VALL
shall be returned (according to the sign of the value), and errno
shall be set to [ERANGE].

If the correct value would cause an underflow, a value whose magnitude
is no greater than the smallest normalized positive
number in the return type shall be returned and errno set to
[ERANGE].

ERRORS

These functions shall fail if:

ERANGE

The value to be returned would cause overflow or underflow.

These functions may fail if:

EINVAL

No
conversion could be performed.

The following sections are informative.

EXAMPLES

None.

APPLICATION USAGE

If the subject sequence has the hexadecimal form and FLT_RADIX is
not a power of 2, and the result is not exactly representable,
the result should be one of the two numbers in the appropriate internal
format that are adjacent to the hexadecimal floating source
value, with the extra stipulation that the error should have a correct
sign for the current rounding direction.

If the subject sequence has the decimal form and at most DECIMAL_DIG
(defined in <float.h>) significant digits, the result should
be correctly rounded. If the subject
sequence D has the decimal form and more than DECIMAL_DIG significant
digits, consider the two bounding, adjacent decimal
strings L and U, both having DECIMAL_DIG significant digits,
such that the values of L, D, and U
satisfy L <= D <= U. The result should be one of
the (equal or adjacent) values that would be obtained
by correctly rounding L and U according to the current
rounding direction, with the extra stipulation that the error
with respect to D should have a correct sign for the current
rounding direction.

The changes to strtod() introduced by the ISO/IEC 9899:1999
standard can alter the behavior of well-formed
applications complying with the ISO/IEC 9899:1990 standard and thus
earlier versions of the base documents. One such example
would be:

an ISO/IEC 9899:1990 standard-compliant library will result in the
function printing:

It's an integer with value 16

With the ISO/IEC 9899:1999 standard, the result is:

It's a float with value 16

The change in behavior is due to the inclusion of floating-point numbers
in hexadecimal notation without requiring that either a
decimal point or the binary exponent be present.

RATIONALE

None.

FUTURE DIRECTIONS

None.

COPYRIGHT

Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of
Electrical and Electronics Engineers, Inc and The Open Group. In the
event of any discrepancy between this version and the original IEEE and
The Open Group Standard, the original IEEE and The Open Group Standard
is the referee document. The original Standard can be obtained online at
http://www.opengroup.org/unix/online.html .